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    console.log(typeof undefined)
    console.log(typeof 1)
    console.log(typeof "a")
    console.log(typeof "")
    console.log(typeof 0)
    console.log(typeof Symbol(1))  // symbol
    console.log(typeof null)  // object
    console.log(typeof [1,2]) // object
    console.log(typeof function(){}) // function 可以识别函数为function
    console.log(null === null) // true
    console.log(null === undefined) // false
    console.log(null === '')
    console.log(null === 0)
    console.log(undefined === undefined) // true
    console.log(undefined === 0)
    console.log(undefined === '')
    let obj 
    obj = {}
    obj = []
    obj = new Date()
    console.log(Object.prototype.toString.call(obj).slice(8, -1).toLowerCase())  




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